A liquid crystal display device is widely used in a display unit or the like of a thin television set or a mobile phone. In relation to performance of the liquid crystal display device, the liquid crystal display device has a good display performance when a display surface is viewed from a front side, but contrast decreases and thus visibility tends to degrade when viewed in an oblique direction. In addition, there is a case where, in grayscale display, grayscale inversion in which brightness is inverted occurs, and thus a display performance is degraded. For this reason, there is a problem in that a so-called viewing angle at which a screen can be observed with favorable visibility is narrow.
Therefore, in the related art, various methods of widening a viewing angle range in which a screen can be observed with favorable visibility have been proposed.
For example, PTL 1 discloses a light diffusion sheet provided with a plurality of unit lenses. The unit lenses in the light diffusion sheet are formed in a one-dimensional direction or two-dimensional directions, each of which includes a total reflection portion in which some incident light is totally reflected at an inner surface, and a high refractive index portion which is made of a material with a predetermined refractive index N1. In addition, a low refractive index portion which is filled with a material having a predetermined refractive index N2 is formed between the unit lenses.
Each unit lens has an approximately trapezoidal shape as a cross-sectional shape; and a lower side of the trapezoid is a light incidence portion, oblique sides form the total reflection portion, and an upper side forms a light emission portion. When an angle formed between an oblique side forming the total reflection portion and a normal to the light emission portion is set to θ, θ is set to have a specific relationship with N1 and N2, the cross sectional shape of the low refractive index portion interposed between the adjacent unit lenses is a substantially triangular shape, an edge of the light incidence portion has a curved shape with a predetermined curvature or is substantially trapezoidal, and the upper side of the light incidence portion has a shape which is parallel to or diagonal with respect to the lower side.
With the light diffusion sheet having this configuration, incident light which is parallel to a normal to a light emission surface is totally reflected at the oblique sides of the unit lens, and thus the light can be emitted toward an observer side without the occurrence of reflection of light on the light emission surface. Therefore, it is considered that a light diffusion sheet with high luminance and contrast can be obtained.